Sickle cell anemia is a single gene disorder affecting the beta globin chain of human adult hemoglobin. Varying phenotypic expressions of this disease have led to studies of genetic factors contributing to this diversity. Factors that lead to stroke and the development of cerebrovascular disease in children with sickle cell disease are not fully understood. This study will determine if common genetic polymorphisms associated with thrombophilia are important risk factors for the development of cerebrobvascular disease and stroke in these children. The genetic polymorphisms to be studied include MTHFR (methylenetetrahydrofolate reductase) variant (C677T mutation), ACE (angiotensin converting enzyme), ID (insertion/deletion) polymorphism, prothrombin 20210 G to A mutation, and mutations in the Factor V gene (Factor V Leiden, Rsa I polymorphisms; in exon 13 of the factor V gene known as R2 and R3 haplotypes, and Factor V R485K polymorphism). Hb SS patients randomized to the STOP study as well as patients screened in STOP II will provide the basis for this study.